It's a tossup for me. I used to love redeyes since I could sleep through from takeoff-landing. Unfortunately these days I don't get as well rested, so the day after the redeye is usually filled with a nap. Day flights are nice to do work, but I always fly economy where there isn't enough room to work on a 15" laptop. Either way, I don't get much work done so it ends up being whatever is cheapest.